Originally Posted by rowan

Another method to reduce spam is to return a temporary error upon first delivery attempt, which will queue the email on the server trying to deliver it. Most email servers will attempt redelivery within a short period (say 30 mins) but bulk emailers don't usually bother. If there is a second attempt to deliver the same mail then the IP is whitelisted and further deliveries are allowed to come through unheeded. Bulk mailers don't try a second time so they're never whitelisted.